Bank card numbers are usually composed of multiple parts, the most common structure is 16 digits (but there are other lengths), generally divided into the following parts: 1. **BIN (Bank Identification Number)**: The first 6 digits represent the identification number of the issuing bank. This number can be used to identify the issuing bank and the country where it is located. For example, certain combinations of digits may indicate a specific bank or financial institution. 2. **Account Number**: The next middle part, usually 7 to 10 digits, is used to identify the specific account of the cardholder. This part of the number is generated by the bank according to internal regulations and may be related to the user's account information. 3. **Check Digit**: The last digit is used to verify the correctness of the entire bank card number, which is usually generated by a method called Luhn algorithm (modulo 10 algorithm). The calculation of the check digit takes into account all the previous digits to ensure the validity of the entire number. ### For example: Suppose there is a bank card number **1234 5678 9012 3456**. - **BIN**: The first 6 digits are `123456`, which represent the issuing bank. - **Account Number**: The next part `789012345` is the specific account identifier under the bank. - **Check Digit**: The last digit `6` is calculated by the Luhn algorithm to ensure that the entire number conforms to specific formatting rules. ### Brief Introduction to Luhn Algorithm: 1. From right to left, mark the even and odd digits. 2. For even digits, multiply by 2: - If the result is greater than 9, add the ones and tens of the result (for example, 8 becomes 16, which becomes 1 + 6 = 7). 3. Add all the digits, including the unchanged odd digits and the processed even digits. 4. If the sum is a multiple of 10, then the number is valid. ### Summary: The composition of bank card numbers is a systematic design designed to ensure that each card number is unique and can effectively identify the issuing bank and account. At the same time, by adding the check bit, the probability of input errors can be effectively reduced.